Surface-modified super absorbent resin and method for preparing same

ABSTRACT

Disclosed herein is a superabsorbent polymer with a surface modified with both a water-soluble polyvalent cationic salt and a polycarbonic acid-based copolymer, wherein an improvement is brought about in processability without significant degradation of other properties and a method is provided for preparing the surface-modified superabsorbent polymer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Korean Patent Application No.10-2014-0183226, filed on Dec. 18, 2014, which is hereby incorporated byreference herein in its entirety.

TECHNICAL FIELD

The present disclosure relates to a surface-modified superabsorbentpolymer resin, and a method for preparing the same. More particularly,the present disclosure relates to a superabsorbent polymer with asurface modified with both a water-soluble polyvalent cationic salt anda polycarbonic acid-based copolymer, wherein an improvement is broughtabout in processability without significant degradation of otherproperties.

BACKGROUND ART

Superabsorbent polymers (SAPs) are synthetic polymer materials having acapacity for absorbing 500 to 1000 times their own weight in moisture.Although developed for practical use in sanitary items such asdisposable diapers for children, sanitary pads, etc., SAPs now findadditional applications in a variety of fields including raw materialsin soil conditioners for horticulture, water stopping agents for civilengineering and construction applications, sheets for raising seedlings,freshness preservatives for food distribution, goods for fomentation,and the like.

Absorption under pressure (AUP) or permeability, which is one of themajor properties of SAPs for use in diapers, may degrade as the SAPs arebroken down during manufacture into diapers. In consideration of thedegradation thereof, the physical properties of SAPs should reachsufficiently high levels. However, various physical properties of SAPsare in a trade-off relation. Due to this limitation, there is thereforea need for the development of a superabsorbent polymer provided withvarious satisfactory physical properties that exist in harmony with eachother.

DISCLOSURE Technical Problem

Leading to the present disclosure, intensive and thorough research intoSAPs resulted in the finding that SAPs, when modified on their surfacewith a water-soluble salt including a polyvalent cationic ion and apolycarbonic acid-based copolymer, improve in physical propertiesincluding permeability, compared to conventional SAPs. It is thereforean object of the present disclosure to provide a superabsorbent polymerresin that is improved in permeability and which retains goodprocessability without degrading other properties, whereby not onlyprocessability in the preparing process of the polymer resin can beimproved sufficiently to decrease a load and to readily control particlesizes and physical properties, but also the property degradationattributed to the breakdown of the resin in applied processing can beminimized, and a method for preparing the same.

Technical Solution

In order to accomplish the above object, an aspect of the presentdisclosure provides a superabsorbent polymer resin with a surfacemodified with a water-soluble polyvalent cationic salt and apolycarbonic acid-based copolymer.

Another aspect of the present disclosure provides a method for preparinga surface-modified superabsorbent polymer, comprising:

a) providing a superabsorbent polymer;

b) pre-treating the superabsorbent polymer of step a) by mixing awater-soluble polyvalent cationic salt in an amount of 0.001 to 5.0parts by weight, based on 100 parts by weight of the superabsorbentpolymer provided in step a); and

c) surface treating the pre-treated superabsorbent polymer of step b) bymixing a polycarbonic acid-based copolymer in an amount of 0.001 to 5.0parts by weight, based on 100 parts by weight of the superabsorbentpolymer provided in step a).

A further aspect of the present disclosure provides a method forpreparing a surface-modified superabsorbent polymer, comprising:

a) providing a superabsorbent polymer;

b) preparing a mixture solution comprising a water-soluble polyvalentcationic salt in an amount of 0.001 to 5.0 parts by weight and apolycarbonic acid-based copolymer in an amount of 0.001 to 5.0 parts byweight, based on 100 parts by weight of the superabsorbent polymerprovided in step a); and

c) surface treating the superabsorbent polymer of step a) with themixture solution of step b).

Advantageous Effects

Compared to conventional SAPs, the SAPs prepared by surface modificationwith a water-soluble polyvalent cationic salt and a polycarbonicacid-based copolymer in accordance with the present disclosure aregreatly improved in permeability. Thus, the SAPs of the presentdisclosure enjoy the advantage of improving in processabilitysufficiently to decrease a load and to readily control particle sizedistribution and physical properties without significant degradation inother properties, and minimizing the property degradation attributed tothe breakdown of the resin in applied processing,

DESCRIPTION OF DRAWINGS

FIG. 1a is a graph showing CRC changes in SAPs between pre- andpost-ball milling (Comparative Example 1, Comparative Example 2, Example1, Example 2, Example 3, Example 4, Example 5, and Example 6 from theleft to the right).

FIG. 1b is a graph showing AUP changes in SAPs between pre- andpost-ball milling (Comparative Example 1, Comparative Example 2, Example1, Example 2, Example 3, Example 4, Example 5, and Example 6 from theleft to the right).

FIG. 1c is a graph showing permeability changes in SAPs between pre- andpost-ball milling (Comparative Example 1, Comparative Example 2, Example1, Example 2, Example 3, Example 4, Example 5, and Example 6 from theleft to the right).

FIG. 2a is a graph showing particle size distributions of SAPs beforeball milling.

FIG. 2b is a graph showing particle size distributions of SAPs afterball milling.

FIG. 3 is a schematic diagram showing structures of polycarbonicacid-based copolymers WRM50, #426, and #427.

BEST MODE

Below, a detailed description will be given of the present disclosure.

In accordance with an aspect thereof, the present disclosure addresses asuperabsorbent polymer with a surface modified with a water-solublepolyvalent cationic salt and a polycarbonic acid-based copolymer.

The water-soluble polyvalent cationic salt plays a role in surfacecrosslinking in the superabsorbent polymer. In some embodiments, thewater-soluble polyvalent cationic salt may be used in an amount of 0.001to 5.0 parts by weight, based on 100 parts by weight of thesuperabsorbent polymer. Within this content range, the water-solublepolyvalent cationic salt allows the superabsorbent polymer to increasein permeability without causing a significant degradation of otherproperties.

In the water-soluble salt, the polyvalent cationic ion may be selectedfrom the group consisting of Al³⁺, Zr⁴⁺, Sc³⁺, Ti⁴⁺, V⁵⁺, Cr³⁺, Mn²⁺,Fe³⁺, Ni²⁺, Cu²⁺, Zn²⁺, Ag⁺, Pt⁴⁺, Au⁺, and a combination thereof whilethe counterpart anion may be selected from the group consisting ofsulfate (SO₄ ²⁻), sulfite (SO₃ ²⁻), nitrate (NO³⁻), metaphosphate(PO³⁻), phosphate (PO₄ ³), and a combination thereof. The water-solublesalt may be particularly aluminum sulfate (Al₂(SO₄)₃) and moreparticularly zirconium sulfate (Zr(SO₄)₂), and may be in the form of ahydrate.

The polycarbonic acid-based copolymer may act as a superplasticizer inthe superabsorbent polymer, and has a structure in which a main chain isconjugated with a plurality of side chains or branches, like a comb, asshown in FIG. 3.

According to some embodiments, the polycarbonic acid-based copolymerconsists of a main chain having a (meth)acrylic acid-based monomer as astructural unit; and a side chain composed of an alkoxypolyalkyleneglycol mono(meth)acrylic acid ester monomer. Preferably, thepolycarbonic acid-based copolymer is contained in an amount of 0.001 to5.0 parts by weight, based on 100 parts by weight of the superabsorbentpolymer. When used after treatment of the superabsorbent polymer withthe water-soluble polyvalent cationic salt, the polycarbonic acid-basedcopolymer in the content range can allow the superabsorbent polymer toimprove in permeability as well as to retain still high processability,without a significant degradation of other properties.

In a particular embodiment, the polycarbonic acid-based copolymer maycontain 50 to 99% by weight of the alkoxypolyalkylene glycolmono(meth)acrylic acid ester monomer and 1 to 50% by weight of the(meth)acrylic acid monomer.

With the monomers in such ranges, the copolymer is advantageous inexerting excellent dispersibility, slump retention, and initialdispersibility, as well as in expressing appropriate air entrainment.

The alkoxypolyalkylene glycol mono(meth)acrylic acid ester monomer thatserves as a side chain of the polycarbonic acid-based copolymer may berepresented by the following Chemical Formula 1:

wherein,

R¹ is a hydrogen atom or methyl;

R²O represents an oxyalkylene moiety of 2 to 4 carbon atoms;

R³ is alkyl of 1 to 4 carbon atoms; and

m is an integer of 50 to 200, expressing an average addition mole numberof oxyalkylene.

When the average addition mole number of oxyalkylene ranges from 50 to200, the side chain guarantees excellent dispersibility and slumpretention. In a particular embodiment, the average addition mole numberof oxyalkylene may be between 50 to 150.

The alkoxy polyalkylene glycol mono(meth)acrylic acid ester monomer maybe at least one selected from the group consisting ofmethoxypolyethylene glycol mono(meth)acrylate, methoxypolypropyleneglycol mono(meth)acrylate, methoxypolybutylene glycolmono(meth)acrylate, methoxypolyethylene glycol polypropylene glycolmono(meth)acrylate, methoxypolyethylene glycol polybutylene glycolmono(meth)acrylate, methoxypolypropylene glycol polybutylene glycolmono(meth)acrylate, methoxypolyethylene glycol polypropylene glycolpolybutylene glycol mono(meth)acrylate, ethoxypolyethylene glycolmono(meth)acrylate, ethoxypolypropylene glycol mono(meth) acrylate,ethoxypolybutylene glycol mono(meth)acrylate, ethoxypolyethylene glycolpolypropylene glycol mono(meth)acrylate, ethoxypolyethylene glycolpolybutylene glycol mono(meth)acrylate, ethoxypolypropylene glycolpolybutylene glycol mono(meth)acrylate, and ethoxypolyethylene glycolpolypropylene glycol polybutylene glycol mono(meth)acrylate.

The (meth)acrylic acid monomer that serves as a structural unit of themain chain of the polycarbonic acid-based copolymer may be representedby the following Chemical Formula 2:

R²—COOM¹  [Chemical Formula 2]

wherein,

R² is an unsaturated hydrocarbon of 2 to 5 carbon atoms; and

M¹ is a hydrogen atom, a monovalent or divalent metal, an ammoniumgroup, or an organic amine group.

The (meth)acrylic acid monomer of Chemical Formula 2 may be at least oneselected from the group consisting of an acrylic acid, a methacrylicacid, a monovalent or divalent metal salt thereof, an ammonium saltthereof, and an organic amine salt thereof.

The polycarbonic acid-based copolymer may be prepared by copolymerizingthe monomers in the presence of a polymerization initiator.Copolymerization may be carried out by solution polymerization or bulkpolymerization, but is not limited thereto.

For example, when polymerization is performed with water as a solvent, awater-soluble polymerization initiator, such as ammonium, alkali metalpersulfate, or hydrogen peroxide may be employed. Solutionpolymerization in a solvent such as a lower alcohol, an aromatichydrocarbon, an aliphatic hydrocarbon, an ester compound or a ketonecompound may employ as a polymerization initiator a peroxide, such asbenzoylperoxide, lauroylperoxide, or cumen hydroperoxide, or an aromaticazo compound such as azobisisobutyronitrile. In this regard, anaccelerator such as an amine compound may be used in combination.

For polymerization in a water-lower alcohol mixture solvent, an eligiblecombination of the various polymerization initiators and optionally theaccelerator may be used.

The polymerization initiator may be used in an amount of 0.5% to 5% byweight, based on the total weight of the monomer mixture employed whilethe polymerization temperature may be set depending on the kind of thesolvent or the polymerization initiator, and may range from 0° C. to120° C.

In order to control the molecular weight of the polycarbonic acid-basedcopolymer, a thiol-based chain transfer agent may be added. Thethiol-based chain transfer agent may be at least one selected from thegroup consisting of mercapto ethanol, thioglycerol, thioglycolic acid,2-mercapto propionic acid, 3-mercapto propionic acid, thiomalic acid,thioglycolic acid octyl, and 3-mercapto propionic acid octyl, and may beused in an amount of 0.01% to 5% by weight, based on the total weight ofthe monomer mixture.

In a particular embodiment, the polycarbonic acid-based copolymer or aneutralized salt thereof may range in weight average molecular weightfrom 30,000 to 70,000 and preferably from 40,000 to 60,000 inconsideration of dispersibility, as measured by GPC (Gel PermeationChromatography).

In some embodiments of the present disclosure, #427 polycarbonicacid-based copolymer, a novel polymer derived from the commerciallyavailable polycarbonic acid-based copolymer WRM 50(LG Chem) byalteration in side chain length and chain density, is used for thesurface modification leading to an improvement in property. Forreference, #427 is identical to WRM50 with the exception that themolecular weight (length) of the alkoxypolyalkylene glycolmono(meth)acrylic acid ester monomer as a side chain is changed.

In Table 1, below, the polycarbonic acid-based copolymers WRM50, #426,and #427 are described for their basic properties including TSC (Totalsolid content:%), pH, and viscosity (25° C.), and results of GPCanalysis are also listed.

TABLE 1 Properties of Three PCEs WRM50, #426, and #427 Basic PropertiesTSC (%) pH Vis. (25° C.) WRM50 50.34 4.9  334 #426 44.71 5.06 307 #42744.12 4.88 204 GPC Result Sample Mw PDI Low-Mw (%) WRM50 40,654 1.9417.7  #426 27,780 2.18 4.2 #427 16,911 1.81 5.9

In addition, as shown in Table 2 and FIG. 3, the polycarbonic acid-basedcopolymers differ in structural features from one to another. The sidechain alkoxypolyalkylene glycol mono(meth)acrylic acid ester monomer islonger (heavier) in #426 and #427 than WRM50, with a higher density ofthe side chain for #427 than #426.

TABLE 2 Density of Polymer Performance Main Chain Side Chain side chainWRM50 Low dispersion Long Short High Low retention #426 High dispersionShort Long Low #427 High dispersion Very short Long High High retention

Further, analysis for the particle size distribution of SAPs showed thatSAPs retained processability even after being subjected to surfacemodification with the polycarbonic acid-based copolymer. It was alsodemonstrated that SAPs were provided with more improved physicalproperties when the surface modification was carried out with acombination of the polycarbonic acid-based copolymer and thewater-soluble polyvalent cationic salt than with the polycarbonicacid-based copolymer alone.

In the preparation of the surface-modified superabsorbent polymer of thepresent disclosure, as described above, the surface modification may beachieved by treating a superabsorbent resin with the water-solublepolyvalent cationic salt, and then with the polycarbonic acid-basedcopolymer (2-step), or by treating a superabsorbent resin with a mixtureof the water-soluble polyvalent cationic salt and the polycarbonicacid-based copolymer (1-step). The surface-modified superabsorbentpolymer, whether prepared in the 2-step process or the 1-step process,is found to still retain high processability.

The method may further comprise milling the surface-modifiedsuperabsorbent polymer, and classifying the milled superabsorbentpolymer by particle size into particles with sizes of less than 150 μm,from 150 μm to less than 300 μm, from 300 μm to less than 600 μm, from600 μm to 850 μm, and greater than 850 μm. The milling step may becarried out with a mill the examples of which include, but are notlimited to, a ball mill, a pin mill, a hammer mill, a screw mill, a rollmill, a disc mill, and a jog mill.

The surface-modified superabsorbent polymer of the present disclosure,when treated with a mixture of the water-soluble polyvalent cationicsalt and the polycarbonic acid-based copolymer, was observed to advancemuch in processability and particularly to exhibit excellentpermeability, compared to conventional SAPs. A great improvement inpermeability is brought into SAPs according to some embodiments of thepresent disclosure, compared to SAPSs lacking both a water-solublepolyvalent cationic salt and a polycarbonic acid-based copolymer.

The SAPs are prepared by a method comprising:

a) polymerizing a monomer composition containing a water-solubleethylenically unsaturated monomer and a polymerization initiator by heator light to give a hydrogel phase polymer;

b) drying the hydrogel phase polymer;

c) milling the dried hydrogel phase polymer into superabsorbent polymerparticles; and

d) adding a surface cross-linking agent to the milled superabsorbentpolymer particles to perform a surface cross-linking reaction.

As used herein, the term “superabsorbent polymer particles” refers toparticles obtained by drying and milling the hydrogel phase polymer. Ingreater detail, the hydrogel phase polymer has a jelly form 1 cm orlarger in size with a high water content (50% or higher) aftercompletion of the polymerization, and is dried and milled into powders,which is termed superabsorbent polymer particles. That is, the hydrogelphase polymer corresponds to a product in a meso-phase of the method.

The method for the preparation of SAPs in accordance with the presentdisclosure starts with a) thermal polymerization or photopolymerizationof water-soluble, ethylenically unsaturated monomers to a hydrogel phasepolymer in the presence of a polymerization initiator.

For this, steps or processes typical in the art may be employed. Indetail, the polymerization initiator contained in the monomercomposition for use in the preparation of the SAPs of the presentdisclosure may depend on the type of polymerization. That is, either aphotopolymerization initiator or a thermal polymerization initiator maybe used. For photopolymerization, however, heat is generated not only byUV light irradiation, but also as the polymerization, which is anexothermic reaction. Hence, a thermal polymerization initiator may beadditionally contained even upon photopolymerization.

Although no special limitations are imparted thereto, the thermalpolymerization initiator useful in the method for the preparation ofSAPs according to the present disclosure may be preferably selected fromthe group consisting of a persulfate salt, an azo compound, hydrogenperoxide, ascorbic acid, and a combination thereof. Examples of theperfulate initiator include sodium persulfate (Na₂S₂O₈), potassiumpersulfate (K₂S₂O₈), and ammonium persulfate ((NH₄)₂S₂O₈). Among the azocompound useful as a thermal polymerization initiator in the preparationof the preparation of SAPs according to the present disclosure are2,2-azobis(2-amidinopropane) dihydrochloride, 2,2-azobis-(N,N-dimethylene)isobutyramidine dihydrochloride,2-(carbamoylazo)isobutyronitrile),2,2-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, and4,4-azobis-(4-cyanovaleric acid).

The photopolymerization initiator available in the method for thepreparation of SAPs according to the present disclosure, although notspecifically limited, may be preferably selected from the groupconsisting of benzoin ether, dialkyl acetophenone, hydroxyl alkylketone,phenyl glyoxylate, benzyl dimethyl ketal, acyl phosphine, α-aminoketone,and a combination thereof. As an acyl phosphine, commercially availablelucirin TPO, that is, 2,4,6-trimethyl-benzoyl-trimethyl phosphine oxidemay be used.

So long as it is typically used in the preparation of SAPs, anywater-soluble, ethylenically unsaturated monomer may be used withoutlimitations in the preparation method of superabsorbent polymer resinsaccording to the present disclosure. Preferably, the water-soluble,ethylenically unsaturated monomer may be selected from the groupconsisting of an anionic monomer or a salt thereof, a non-ionichydrophilic monomer, an amino group-containing unsaturated monomer and aquaternary salt thereof, and a combination thereof. Examples of thewater-soluble, ethylenically unsaturated monomer include anionicmonomers or salts thereof, such as acrylic acid, methacrylic acid,anhydrous maleic acid, fumaric acid, crotonic acid, itaconic acid,2-acryloylethanesulfonic acid, 2-methacryloylethanesulfonic acid,2-(meth)acryloylpropanesulfonic acid, and2-(meth)acrylamide-2-methylpropane sulfonic acid; non-ionic hydrophilicmonomers, such as (meth)acrylamide, N-substituted (meth)acrylate,2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate,methoxypolyethylene glycol (meth)acrylate, and polyethylene glycol(meth)acrylate; and an amino group-containing unsaturated monomers orquaternary salts thereof, such as (N,N)-dimethylaminoethyl(meth)acrylate, and (N,N)-dimethylaminopropyl (meth)acrylamide, withpreference for an acrylic acid or a salt thereof. Advantageously fromacrylic acid or a salt thereof, SAPs that are particularly improved inabsorbency can be obtained.

For resource recycling, micro- or submicro-particles of the preparedSAPs, that is, the prepared SAPs with a particle size of less than 150μm, may be contained in the monomer composition during the preparationof SAPs according to the present disclosure. In detail, the polymerparticles with a particle size of less than 150 μm may be added to themonomer composition before the polymerization reaction or to thereaction mixture at an initial, middle or late phase of thepolymerization. No limitations are imparted to the amount of thesuperabsorbent polymer powder. Preferably, it is added in an amount of 1to 10 parts by weight, based on 100 parts by weight of the monomercomposition, in terms of preventing physical properties of the finalSAPs from deteriorating.

In the method for preparing SAPs in accordance with the presentdisclosure, the content of the water-soluble ethylenically unsaturatedmonomer in the monomer composition may be properly determined inconsideration of polymerization time and reaction conditions, and maypreferably range from 40 to 55% by weight. Less than 40% by weight ofthe water-soluble ethylenically unsaturated monomer is economicallydisadvantageous. When the monomer is used in an amount exceeding 55% byweight, the resulting hydrogel phase polymer may be milled at a lowrate.

So long as it is typically used for thermal polymerization orphotopolymerization in the art, any technique may be applied withoutlimitations to the preparation of a hydrogel phase polymer from themonomer composition. Largely, polymerization is divided into thermalpolymerization and photopolymerization according to energy source. Onthe whole, thermal polymerization may be performed in a reactorinstalled with a stirring shaft, such as a kneader. Forphotopolymerization, a conveyer belt may run under a light source in areactor. These techniques are illustrated as exemplary embodiments, butare not to be construed to limit the present disclosure.

For example, a hydrogel phase polymer is prepared in a reactor installedwith a stirring shaft, such as a kneader, by thermal polymerization,e.g., by providing hot air to the reactor or by heating the reactor, anddischarged from the reactor as particles millimeters to centimeters longaccording to the type of the stirring shaft. In detail, the size of theobtained hydrogel phase polymer particles may vary depending on theconcentration and feeding rate of the monomer composition, and typicallyranges from 2 to 50 mm.

In addition, when photopolymerization is performed on a movable conveyerbelt as mentioned above, the resulting hydrogel phase polymer maytypically take a sheet-like form with a width equal to that of the belt.The polymer sheet may vary in thickness depending on the concentrationand feeding rate of the monomer composition. The monomer composition ispreferably fed such that a sheet-like polymer with a thickness of 0.5 to5 cm may be obtained. A feeding condition of the monomer compositionthat affords too thin a polymer sheet may result in low productivity.When the thickness of the sheet-like polymer exceeds 5 cm, thepolymerization reaction may not occur evenly over the full thickness.

The light source available in the photopolymerization step is notimparted with special limitations. Any UV light that causes aphotopolymerization reaction may be employed. For example, light with awavelength of 200 to 400 nm, or UV radiation such as that from a Xelamp, a mercury lamp, or a metal halide lamp may be used. Thephotopolymerization may be performed for approximately 5 sec toapproximately 10 min under a light intensity of approximately 0.1 mw/cm²to approximately 1 kw/cm². When the light intensity is too low or theirradiation time is too short, insufficient polymerization reactions mayresult. On the other hand, too high a light intensity or too long anirradiation time may cause a poor quality of the superabsorbentpolymers.

Next, step b) of drying the hydrogel phase polymer is carried out in themethod for preparing SAPs in accordance with the present disclosure.

The hydrogel phase polymer obtained in step a) has a water content of 30to 60% by weight. As used herein, the term “water content” refers toweight percentage of water to the total weight of the hydrogel phasepolymer. The amount of water may be obtained by subtracting the weightof dried polymer from the total weight of the hydrogel phase polymer (indetail, after the polymer is dried by infrared heating, the mass lossattributed to moisture evaporation is measured. The drying condition issuch that the atmosphere is heated from room temperature to 180° C. andmaintained at 180° C., with a total drying time set to be 20 minincluding 5 min for the temperature increment).

The hydrogel phase polymer obtained in step a) undergoes a dryingprocess. Preferably, the drying may be conducted at 150° C. to 250° C.The term “drying temperature”, as used herein, means the temperature ofa heat medium provided for drying or the temperature of a dryerincluding a heat medium and the polymer therein.

A drying temperature of less than 150° C. may make the drying time long,and is apt to degrade properties of the final SAPs. When the dryingtemperature exceeds 250° C., there is high likelihood that only thesurface of the polymer is dried, which leads to the generation of finepowder in a subsequent milling step, and the degradation of propertiesof the final SAPs. Preferably, the drying may be conducted at 150° C. to250° C., and more particularly at 160° C. to 200° C.

As for the drying time, its configuration is not specifically limited,and may be set to range from 20 to 90 min in consideration of processefficiency.

Any drying process that is typically used to dry hydrogel phase polymersmay be selected, without limitations to the configuration thereof. Indetail, the drying step may be conducted by supplying hot air, orirradiating with IR light, microwaves, or UV light. After the dryingstep, the water content of the polymer may be decreased to 0.1 to 10% byweight.

In advance of the drying step, as needed, the method for preparing SAPsin accordance with the present disclosure may further comprise brieflycrushing the hydrogel phase polymer to enhance the efficiency of thedrying step. In this brief crushing step, the hydrogel phase polymer maybe crushed into particles with a size of 1 mm to 15 mm. It istechnically difficult to crush the polymer into particles less than 1 mmin size due to the high water content of the hydrogel phase polymer.Even though possible to crush the polymer into particles less than 1 mmin size, the crushed particles are prone to agglomeration therebetween.On the other hand, crushed particles with a size of greater than 15 mmdo not guarantee the subsequent drying step will be efficient.

For use in the brief crushing step prior to the drying step, a crushingmachine may be employed without limitations to the configurationthereof. Examples of the crushing machine include, but are not limitedto, a vertical pulverizer, a turbo cutter, a turbo grinder, a rotarycutter mill, a cutter mill, a disc mill, a shred crusher, a crusher, achopper, and a disc cutter.

When a crushing step is carried out to enhance the drying efficiency inthe subsequent drying step, the hydrogel phase polymer may be likely toadhere to the surface of the crusher due to its high water content. Toincrease the efficiency of the pre-drying crushing step, an additivepreventive of the adherence of the hydrogel phase polymer to the crushermay be employed. Examples of the additive available for preventing theadherence include a powder aggregation preventer such as steam, water, asurfactant, or inorganic powder, e.g., clay or silica; a thermalpolymerization initiator, such as a persulfate initiator, an azo-typeinitiator, hydrogen peroxide, and ascorbic acid; a crosslinking agent,such as an epoxy-based crosslinking agent, a diol-containingcrosslinking agent, a crosslinking agent containing acrylate ofmultifunctionality, e.g., bi- or tri-functionality, and amono-functional compound containing a hydroxide group, but are notlimited thereto.

After the drying step, the method for preparing SAPs according to thepresent disclosure proceeds to c) milling the dried hydrogel phasepolymer into particles. The polymer particles obtained in the millingstep have a particle size of 150 to 850 μm. The milling step of themethod for preparing SAPs according to the present disclosure may beachieved with a mill the examples of which include, but are not limitedto, a pin mill, a hammer mill, a screw mill, a roll mill, a disc milland a jog mill.

Next, the method for preparing SAPs in accordance with the presentdisclosure goes with d) adding a surface crosslinking agent to themilled hydrogel phase polymer to perform a surface crosslinkingreaction. The surface crosslinking agents added to the superabsorbentpolymer particles may have the same composition irrespective of particlesizes or may be, as needed, different in composition by particle size.

Any surface crosslinking agent that reacts with a functional group ofthe polymer can be employed without limitations to the configurationthereof in the method for preparing SAPs according to the presentdisclosure. Preferably to enhance properties of the SAPs thus prepared,the surface crosslinking agent may be selected from the group consistingof a polyhydric compound; an epoxy compound; a polyamine compound; ahaloepoxy compound; a haloepoxy compound condensate; an oxazolinecompound; a mono-, di- or polyoxazolidinone compound; a cyclic ureacompound; a multi-valent metal salt; an alkylene carbonate compound; anda combination thereof.

Concrete examples of the polyhydric alcohol compound include mono-, di-,tri-, tetra- or polyethylene glycol, monopropylene glycol,1,3-propanediol, dipropylene glycol, 2,3,4-trimethyl-1,3-pentanediol,polypropylene glycol, glycerol, polyglycerol, 2-butene-1,4-diol,1,4-butanediol, 1,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, and1,2-cyclohexanedimethanol.

The epoxy compound may be ethylene glycol diglycidyl ether or glycidol.The polyamine compound may be selected from the group consisting ofethylene diamine, diethylene triamine, triethylene tetramine,tetraethylene pentamine, pentaethylene hexamine, polyethylene imine,polyamide polyamine, and a combination thereof.

Epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin may fallwithin the scope of the haloepoxy compound useful as a surfacecrosslinking agent. The mono-, di- or polyoxazolidinone compound may beexemplified by 2-oxazolidinone. Ethylene carbonate may be representativeof the alkylene carbonate compound. These compounds may be used alone orin combination. In order to enhance the efficiency of the surfacecrosslinking process, the surface crosslinking agent preferably includesat least one polyhydric alcohol compound, and more preferably apolyhydric alcohol compound of 2 to 10 carbon atoms.

The amount of the surface crosslinking agent added to the surface of thepolymer particles may be determined according to the type of the surfacecrosslinking agent or the reaction condition, but may typically rangefrom 0.001 to 5 parts by weight, based on 100 parts by weight of thepolymer, preferably from 0.01 to 3 parts by weight, and more preferablyfrom 0.05 to 2 parts by weight.

If too little the surface crosslinking agent is used, the surfacecrosslinking reaction may not occur. On the other hand, the presence ofthe surface crosslinking agent in an amount exceeding 5 parts by weightbased on 100 parts by weight of the polymer induces an excessive surfacecrosslinking reaction, rather degrading physical properties of the SAPs.

No limitations are imposed on the modality of adding the surfacecrosslinking agent to the polymer. For example, the surface crosslinkingagent may be mixed with the polymer powder in a reactor, sprayed on thepolymer powder, or fed, together with the polymer, to a reactor thatcontinuously operates, such as a mixer.

In the step of adding a surface crosslinking agent, the polymerpreferably has a surface temperature of 60 to 90° C.

To complete the temperature elevation to a reaction temperature within 1to 60 min after the addition of the surface crosslinking agent, thepolymer itself may preferably have a temperature of 20° C. to 80° C.upon the addition of the surface crosslinking agent. To maintain thetemperature in the polymer itself, a process subsequent to the dryingstep, which proceeds at a relatively high temperature, may be run withina short period of time, without delay. When the subsequent process isdifficult to complete within a short period of time, the polymer may beseparately heated.

Also, to complete the temperature elevation to a reaction temperaturewithin 1 to 60 min after the addition of the surface crosslinking agent,the surface crosslinking agent itself added to the polymer may beheated.

When a surface crosslinking reaction is conducted after a temperatureelevation for the surface crosslinking reaction is achieved within 1 to60 min, the surface crosslinking process may be run efficiently. Thus,the SAPs thus prepared can exhibit excellent physical properties, with aminimum residual monomer content therein. The temperature of the surfacecrosslinking agent may be preferably adjusted to a range of 5° C. to 60°C., and more preferably 10° C. to 40° C. When the temperature of thesurface crosslinking agent is below 5° C., the effect of the elevatedtemperature of the crosslinking agent on the reduced time of elevationto a surface crosslinking reaction temperature is offset. On the otherhand, the surface crosslinking agent heated above 60° C. may not beevenly dispersed over the polymer particles. As used herein, the term“surface crosslinking reaction temperature” is defined as the overalltemperature of the surface crosslinking agent and the polymer used inthe surface crosslinking reaction.

Without limitations, a temperature elevating means for the surfacecrosslinking reaction may be employed. By way of example, a heat mediummay be provided, or the reaction mixture may be directly heated withelectricity. As a heat source, steam, electricity, UV light, or IRradiation may be used, or heated thermal liquid may be employed.

In the method for preparing SAPs according to the present disclosure,the crosslinking reaction after the completion of temperature elevationmay be run for 1 to 60 min, preferably for 5 min to 40 min, and mostpreferably for 10 min to 20 min. A reaction time shorter than 1 min doesnot guarantee a sufficient crosslinking reaction. When a crosslinkingreaction time exceeds 60 min, the surface crosslinking reaction proceedsexcessively, resulting in degradation of the physical properties of theSAPs, and a breakdown of the polymer due to the lengthy retention in thereactor.

In accordance with another aspect thereof, the present disclosureaddresses a method for preparing a surface-modified superabsorbentpolymer, comprising:

a) providing a superabsorbent polymer;

b) pre-treating the superabsorbent polymer of step a) by mixing awater-soluble polyvalent cationic salt in an amount of 0.001 to 5.0parts by weight, based on 100 parts by weight of the superabsorbentpolymer provided in step a); and

c) surface treating the pre-treated superabsorbent polymer of step b) bymixing a polycarbonic acid-based copolymer in an amount of 0.001 to 5.0parts by weight, based on 100 parts by weight of the superabsorbentpolymer provided in step a).

Also contemplated in accordance with a further aspect of the presentdisclosure is a method for preparing a surface-modified superabsorbentpolymer, comprising:

a) providing a superabsorbent polymer;

b) preparing a mixture solution comprising a water-soluble polyvalentcationic salt in an amount of 0.001 to 5.0 parts by weight and apolycarbonic acid-based copolymer in an amount of 0.001 to 5.0 parts byweight, based on 100 parts by weight of the superabsorbent polymerprovided in step a); and

c) surface treating the superabsorbent polymer of step a) with themixture solution of step b).

Whether prepared in the 2-step process wherein the surface modificationis achieved by treating a superabsorbent resin with the water-solublepolyvalent cationic salt, and then with the polycarbonic acid-basedcopolymer, or in the 1-step process wherein the surface modification isachieved by treating a superabsorbent resin with a mixture of thewater-soluble polyvalent cationic salt and the polycarbonic acid-basedcopolymer, the surface-modified superabsorbent polymer is found to stillretain high processability.

The method may further comprise milling the surface-modifiedsuperabsorbent polymer, and classifying the milled superabsorbentpolymer by particle size into particles with sizes of less than 150 μm,from 150 μm to less than 300 μm, from 300 μm to less than 600 μm, from600 μm to 850 μm, and greater than 850 μm. The milling step may becarried out with a mill the examples of which include, but are notlimited to, a ball mill, a pin mill, a hammer mill, a screw mill, a rollmill, a disc mill, and a jog mill.

The water-soluble polyvalent cationic salt plays a role in surfacecrosslinking in the superabsorbent polymer. In some embodiments, thewater-soluble polyvalent cationic salt may be used in an amount of 0.001to 5.0 parts by weight, based on 100 parts by weight of thesuperabsorbent resin. Within this content range, the water-solublepolyvalent cationic salt can allow the superabsorbent polymer toincrease in permeability without causing a significant degradation ofother properties.

In the water-soluble salt, the polyvalent cationic ion may be selectedfrom the group consisting of Al³⁺, Zr⁴⁺, Sc³⁺, Ti⁴⁺, V⁵⁺, Cr³⁺, Mn²⁺,Fe³⁺, Ni²⁺, Cu²⁺, Zn²⁺, Ag⁺, Pt⁴⁺, Au⁺, and a combination thereof whilethe counterpart anion may be selected from the group consisting ofsulfate (SO₄ ²⁻), sulfite (SO₃ ²⁻), nitrate (NO³⁻), metaphosphate(PO³⁻), phosphate (PO₄ ³), and a combination thereof. The water-solublesalt may be particularly aluminum sulfate (Al₂(SO₄)₃) and moreparticularly zirconium sulfate (Zr(SO₄)₂), and may be in the form of ahydrate.

The polycarbonic acid-based copolymer may act as a superplasticizer inthe superabsorbent polymer, and has a structure in which a main chain isconjugated with a plurality of side chains or branches, like a comb, asshown in FIG. 3.

According to some embodiments, the polycarbonic acid-based copolymerconsists of a main chain having a (meth)acrylic acid-based monomer as astructural unit; and a side chain composed of an alkoxypolyalkyleneglycol mono(meth)acrylic acid ester monomer. Preferably, thepolycarbonic acid-based copolymer is contained in an amount of 0.001 to5.0 parts by weight, based on 100 parts by weight of the superabsorbentpolymer. When used after treatment of the superabsorbent polymer withthe water-soluble polyvalent cationic salt, the polycarbonic acid-basedcopolymer in the content range allows the superabsorbent polymer toimprove in permeability as well as to retain still high processability,without a significant degradation of other properties.

In a particular embodiment, the polycarbonic acid-based copolymer maycontain 50 to 99% by weight of the alkoxypolyalkylene glycolmono(meth)acrylic acid ester monomer and 1 to 50% by weight of the(meth)acrylic acid monomer.

With the monomers in such ranges, the copolymer is advantageous to exertexcellent dispersibility, slump retention, and initial dispersibility,and to express appropriate air entrainment.

The alkoxypolyalkylene glycol mono(meth)acrylic acid ester monomer thatserves as a side chain of the polycarbonic acid-based copolymer may berepresented by the following Chemical Formula 1:

wherein,

R¹ is a hydrogen atom or methyl;

R²O represents an oxyalkylene moiety of 2 to 4 carbon atoms;

R³ is alkyl of 1 to 4 carbon atoms; and

m is an integer of 50 to 200, expressing an average addition mole numberof oxyalkylene.

When the average addition mole number of oxyalkylene ranges from 50 to200, the side chain guarantees excellent dispersibility and slumpretention. In a particular embodiment, the average addition mole numberof oxyalkylene may be between 50 to 150.

The alkoxy polyalkylene glycol mono(meth)acrylic acid ester monomer maybe at least one selected from the group consisting ofmethoxypolyethylene glycol mono(meth)acrylate, methoxypolypropyleneglycol mono(meth)acrylate, methoxypolybutylene glycolmono(meth)acrylate, methoxypolyethylene glycol polypropylene glycolmono(meth)acrylate, methoxypolyethylene glycol polybutylene glycolmono(meth)acrylate, methoxypolypropylene glycol polybutylene glycolmono(meth)acrylate, methoxypolyethylene glycol polypropylene glycolpolybutylene glycol mono(meth)acrylate, ethoxypolyethylene glycolmono(meth)acrylate, ethoxypolypropylene glycol mono(meth)acrylate,ethoxypolybutylene glycol mono(meth)acrylate, ethoxypolyethylene glycolpolypropylene glycol mono(meth)acrylate, ethoxypolyethylene glycolpolybutylene glycol mono(meth)acrylate, ethoxypolypropylene glycolpolybutylene glycol mono(meth)acrylate, and ethoxypolyethylene glycolpolypropylene glycol polybutylene glycol mono(meth)acrylate.

The (meth)acrylic acid monomer that serves as a structural unit of themain chain of the polycarbonic acid-based copolymer may be representedby the following Chemical Formula 2:

R²—COOM¹  [Chemical Formula 2]

wherein,

R² is an unsaturated hydrocarbon of 2 to 5 carbon atoms; and

M¹ is a hydrogen atom, a monovalent or divalent metal, an ammoniumgroup, or an organic amine group.

The (meth)acrylic acid monomer of Chemical Formula 2 may be at least oneselected from the group consisting of an acrylic acid, a methacrylicacid, a monovalent or divalent metal salt thereof, an ammonium saltthereof, and an organic amine salt thereof.

The polycarbonic acid-based copolymer may be prepared by copolymerizingthe monomers in the presence of a polymerization initiator.Copolymerization may be carried out by solution polymerization or bulkpolymerization, but is not limited thereto.

For example, when polymerization is performed with water as a solvent, awater-soluble polymerization initiator, such as ammonium, alkali metalpersulfate, or hydrogen peroxide, may be employed. Solutionpolymerization in a solvent such as a lower alcohol, an aromatichydrocarbon, an aliphatic hydrocarbon, an ester compound or a ketonecompound may employ as a polymerization initiator a peroxide, such asbenzoylperoxide, lauroylperoxide, or cumen hydroperoxide, or an aromaticazo compound such as azobisisobutyronitrile. In this regard, anaccelerator such as an amine compound may be used in combination.

For polymerization in a water-lower alcohol mixture solvent, an eligiblecombination of the various polymerization initiators and optionally theaccelerator may be used.

The polymerization initiator may be used in an amount of 0.5% to 5% byweight, based on the total weight of the monomer mixture employed whilethe polymerization temperature may be set depending on the kind of thesolvent or the polymerization initiator, and may range from 0° C. to120° C.

In order to control the molecular weight of the polycarbonic acid-basedcopolymer, a thiol-based chain transfer agent may be added. Thethiol-based chain transfer agent may be at least one selected from thegroup consisting of mercapto ethanol, thioglycerol, thioglycolic acid,2-mercapto propionic acid, 3-mercapto propionic acid, thiomalic acid,thioglycolic acid octyl, and 3-mercapto propionic acid octyl, and may beused in an amount of 0.01% to 5% by weight, based on the total weight ofthe monomer mixture.

In a particular embodiment, the polycarbonic acid-based copolymer or aneutralized salt thereof may range in weight average molecular weightfrom 30,000 to 70,000 and preferably from 40,000 to 60,000 inconsideration of dispersibility, as measured by GPC (Gel PermeationChromatography).

According to some embodiments of the present disclosure, #427polycarbonic acid-based copolymer, a novel polymer derived from thecommercially available polycarbonic acid-based copolymer WRM 50(LG Chem)by alteration in side chain length and chain density, is used for thesurface modification leading to an improvement in property. Forreference, #427 is identical to WRM50 with the exception that themolecular weight (length) of the alkoxypolyalkylene glycolmono(meth)acrylic acid ester monomer as a side chain is changed.

In Table 1, the polycarbonic acid-based copolymers WRM50, #426, and #427are described for their basic properties including TSC (Total solidcontent:%), pH, and viscosity (25° C.), and results of GPC analysis arealso listed.

In addition, as shown in FIG. 3, the polycarbonic acid-based copolymersdiffer in structural features from one to another. The side chainalkoxypolyalkylene glycol mono(meth)acrylic acid ester monomer is longer(heavier) in #426 and #427 than WRM50, with a higher density of sidechain for #427 than #426

Further, analysis for the particle size distribution of SAPs showed thatSAPs retained processability even after being subjected to surfacemodification with the polycarbonic acid-based copolymer. It was alsodemonstrated that SAPs were provided with further improved physicalproperties when the surface modification was carried out with acombination of the polycarbonic acid-based copolymer and thewater-soluble polyvalent cationic salt than with the polycarbonicacid-based copolymer alone.

With regard to other descriptions of the SAPs of the preparation methodaccording to the present disclosure, reference may be made to thesurface-modified SAPs according to the present disclosure.

MODE FOR INVENTION

A better understanding of the present disclosure may be obtained throughthe following examples that are set forth to illustrate, but are not tobe construed as limiting the present disclosure. While specificembodiments of and examples for the invention are described above forillustrative purposes, various equivalent modifications are possiblewithin the scope of the invention, as those skilled in the relevant artwill recognize. In addition, unless stated otherwise, the terms “%” and“part”, as used in the context of amount, are on the basis of mass.

Preparation of Hydrogel Phase Polymer and Superabsorbent Polymer

A monomer mixture was obtained by mixing 100 g of acrylic acid, 0.30 gof polyethylene glycol diacrylate as a crosslinking agent, 0.033 g ofdiphenyl(2,4,6-trimethylbenzoyl)-phosphine oxide as an initiator, 38.9 gof caustic soda (NaOH), and 103.9 g of water. The monomer mixture wasfed onto a continuously moving conveyer belt, and subjected topolymerization for 2 min under UV light (intensity: 2 mW/cm²) to obtaina hydrogel phase polymer. The hydrogel phase polymer obtained inPreparation Example 1 was cut into a size of 5×5 mm, dried for 2 hrs at170° C. in a hot air dryer, milled using a pin mill, and screened with asieve to give superabsorbent polymer particles with a size of 150 to 850μm.

Preparation of Polycarbonic Acid-Based Copolymer #427

To a 2-L glass reactor equipped with a thermometer, a stirrer, adropping funnel, a nitrogen inlet, and a reflux condenser was added 200parts by weight of water, and the reactor was purged with nitrogen gasand heated to 75° C. while the water was stirred.

After 20 parts by weight of an aqueous 3% by weight ammonium persulfatesolution was added and slowly dissolved in the reactor, an aqueousmonomer solution containing 400 parts by weight of methoxypolyethyleneglycol monomethacrylate (average addition mole number of ethylene oxideof 100), 40 parts by weight of acrylic acid, 40 parts by weight ofmethacrylic acid, and 90 parts by weight of water, a mixture solution of3.0 parts by weight of 2-mercapto ethanol and 30 parts by weight ofwater, and 70 parts by weight of an aqueous 3% by weight ammoniumpersulfate solution were dropwise added over 4 hrs. Thereafter, 10 partsby weight of an aqueous 3% by weight ammonium persulfate solution wasadded at once. Then, the solution was maintained at 75° C. for 1 hr.

After completion of the polymerization, the reaction mixture was cooledto room temperature, and neutralized for about 1 hr with an aqueous 30%by weight sodium hydroxide solution to afford a water-soluble copolymersalt with a solid content of 45%. The water-soluble copolymer salt wasfound to have a weight average molecular weight of 40,000 as measured byGPC (Gel Permeation Chromatography).

Preparation Example 1

In a high-speed mixer, 250 g of the superabsorbent polymer was mixed at1000 rpm for 20 sec, and 6.25 g of a 25% by weight aqueous solution ofAl₂(SO₄)₃(14-18 hydrate)(Junsei) or Zr(SO₄)₂(4 hydrate)(Alfa aesar) wasinjected into the mixer through a plastic syringe, followed by mixingfor an additional 3 min. Then, 15 g of an 11% by weight aqueous solutionof polycarbonic acid-based copolymer #427(LG Chem) was injected througha plastic syringe, and mixed for an additional 3 min, and the high-speedmixer was stopped. The resulting superabsorbent polymer was aged for 30min at room temperature while stirring, and then screened with a sieveto give superabsorbent polymer particles with a size of 300 to 600 μm.

Preparation Example 2

In a high-speed mixer, 250 g of the superabsorbent polymer was mixed at1000 rpm for 20 sec, and 6.25 g of a 25% by weight aqueous solution ofAl₂(SO₄)₃(14-18 hydrate) and 15 g of a 11% by weight aqueous solution ofpolycarbonic acid-based copolymer #427(LG Chem) were injected into themixer through a plastic syringe, followed by mixing for an additional 3min After the high-speed mixer was stopped, the resulting superabsorbentpolymer was aged for 30 min at room temperature while stirring, and thenscreened with a sieve to give superabsorbent polymer particles with asize of 300 to 600 μm.

Preparation Example 3

In a ball mill, 20.0 g of a superabsorbent polymer with a size of 300 to600 μm (a conventional superabsorbent polymer or the polymer ofPreparation Example 1 or 2) was milled for 20 min with 10 ceramic balls.The milled polymer was screened with a standard sieve to givesuperabsorbent polymer particles with a size of 300 to 600 μm.

TABLE 3 #427 Polymer with Surface Modified with Al₂(SO₄)₃•14-18H₂O orZr(SO₄)₂•4H₂O Ball # of Polyvalent Cationic Salt PCE Ex.# Mill StepsPreparation Species Amount (g) H₂O (g) Name Amount (g) H₂O (g) C.1Before — — — — — — — — C2 After Prep. Ex. 3 1 Before 2-Step Prep. Ex. 3Al₂(SO₄)₃* 1.5625 4.6875 #427 1.654 13.346 2 After Prep. Ex. 1&3 3Before 1-Step Prep. Ex. 2 4 After Prep. Ex. 2&3 5 Before 2-Step Prep.Ex. 3 Zr(SO₄)₂** 6 After Prep. Ex. 1&3 *Al₂(SO₄)₃•14-18H₂O**Zr(SO₄)₂•4H₂O

TABLE 4 Physical Properties CRC AUP Permeability Particle SizeDistribution (PSD, %) Ex.# (g/g) (g/g) (sec) >850 μm 600-850 μm 300-600μm 150-300 μm <150 μm C.1 35.0 24.2 660 0.06 29.08 43.52 24.13 3.21 C235.5 22.4 1202 0.00 1.01 91.51 5.81 1.67 1 32.8 19.2 185 0.31 29.2744.63 24.11 1.69 2 33.5 18.2 242 0.00 0.96 87.98 9.60 1.46 3 32.7 20.3598 0.02 32.22 45.57 20.40 1.79 4 33.5 20.4 560 0.00 2.53 91.87 4.750.86 5 32.7 18.8 62 0.06 34.82 45.41 18.49 1.22 6 33.1 19.3 59 0.05 3.5790.79 5.08 0.50

Test Examples: Assay for Physical Properties Test Example 1: ParticleSize Distribution

The SAPs prepared in Preparation Examples 1 to 3 were measured forparticle size distribution. The measurement was conducted according tothe EDANA-recommended method WSP 240.3. For this, the SAP particlesprepared in Examples 1 to 6 and Comparative Examples 1 and 2 were placedin an amount of 100 g on each of 850 μm, 600 μm, 300 μm, and 150 μm panmeshes, and vibrated for 10 min with an amplitude of 1.00 mm. The amountof the particles held up on each sieve was measured to calculate thecontent as a percentage.

Test Example 2: Centrifugal Retention Capacity (CRC)

Each of the SAPs prepared in Preparation Examples 1 to 3 was measuredfor centrifugal retention capacity (CRC). The measurement of CRC wascarried out according the EDANA (European Disposables and NonwovensAssociation)-recommended method WSP 241.3(10) (IST 241.2(02). W grams(about 0.1 g) of each of the SAPs with a particle size of 300 to 600 μm,prepared in Examples 1 to 6 and Comparative Examples 1 and 2, was placedinto a non-woven bag that was then sealed, and immersed into 0.9 mass %saline. After 30 min of immersion, centrifugation was carried out at250×g for 3 min for dewatering. The dewatered bag alone weighed W2 (g).The same procedure was repeated, with the exception that no resins wereemployed. In this regard, the dewatered bag alone weighed W1 (g). CRC(g/g) was calculated from the weight measurements according to thefollowing equation.

CRC(g/g)={(W2(g)−W1(g))/W(g)}−1  [Math Equation 1]

Test Example 3: Absorption Under Pressure (AUP)

Each of the SAPs prepared in Preparation Examples 1 to 3 was measuredfor absorption under pressure (AUP). The measurement of AUP was carriedout according the EDANA(European Disposables and NonwovensAssociation)-recommended method WSP 242.3 (11) (IST 242.2(02)). Briefly,a stainless-steel 400 mesh net was welded onto the bottom of a plasticsupporting cylinder having an internal diameter of 60 mm. Then, 0.90 gof each of the SAPs with a particle size of 300 to 600 μm, prepared inExamples 1 to 6 and Comparative Examples 1 and 2, was evenly sprayedonto that metal net on the bottom at room temperature and a RH of 50%. Apiston was placed so as to evenly impart a load of 4.83 kPa (0.7 psi) tothe sprayed resin. The piston had an external diameter slightly lessthan 60 mm such that it smoothly moved vertically within the cylinder,with no spaces left between the external wall of the piston and theinternal wall of the cylinder. The resulting cylinder weighed Wa (g).

A 5-mm thick glass filter with a diameter of 90 mm was placed in a150-mm petri dish. A physiological saline containing 0.90% by weight ofsodium chloride was added until it was flush with the top side of theglass filter. A sheet of filter paper with a diameter of 90 mm wasplaced on the saline. Then, a measuring device set was placed on the wetfilter paper so that the paper could absorb the solution under the load.An hour later, the measuring device set was lifted, and the resultingcylinder weighed Wb (g).

Absorbency under pressure was calculated from the measurements Wa and Wbaccording to the following equation:

AUP(g/g)=[Wb(g)−Wa(g)]/Wt. of absorbent resin (g)  [Math Equation 2]

Test Example 4: Permeability

Each of the SAPs prepared in Preparation Examples 1 to 3 was measuredfor permeability. Briefly, 0.2 g of each of the SAPs with a particlesize of 300 to 600 μm, prepared in Examples 1 to 6 and ComparativeExamples 1 and 2, was placed in a 20-mm diameter cylinder installed onthe bottom with a glass filter and a stopcock, and 50 g of 0.9% salinewas poured along the wall so that the SAPs were washed down. After 30min, the SAPs were pressed for 1 min using a piston loaded with abalance (a total of 0.3 psi) before the stopcock was opened.Permeability was determined by measuring the time taken for the surfaceof the saline to reach the marked arrival line (the surface level of 20ml saline in the cylinder under the piston) from the marked start line(the surface level of 40 ml saline in the cylinder under the piston)using a stopwatch.

As is understood from data of Table 4, the SAPs having the surface ofwhich was treated with a mixture of the water-soluble polyvalentcationic salt and the polycarbonic acid-based copolymer in accordancewith the present disclosure were found, particularly after ball milling,to increase in particle size distribution from 300 μm to less than 600μm and to decrease in particle size distribution from 150 μm to lessthan 300 μm and from 600 μm to less than 850 μm. With an increase in theparticle size distribution from 300 μm to less than 600 μm, the SAPswere more homogeneously mixed with the surface crosslinking agent toimprove both the absorption properties (CRC, AUP) and permeability,which resulted in excellent processability, compared to conventionalSAPs. Particularly, excellent effects on permeability were detected. TheSAPs according to some embodiments of the present disclosure exhibitedgreatly improved permeability, compared to SAPs that were not treatedwith the water-soluble polyvalent cationic salt and the polycarbonicacid-based copolymer. In addition, the SAPs of the present disclosurewere unlikely to change in permeability even after ball milling,indicating that the SAPs of the present disclosure are likely to retainphysical properties even under external pressures and impacts such asphysical breakdown. Taken together, the data obtained above demonstratethat the surface modification according to the present disclosureensures excellent permeability as well as processability for the SAPswithout significantly degrading other properties.

1. A surface-modified superabsorbent polymer, having a surface modifiedwith a water-soluble polyvalent cationic salt and a polycarbonicacid-based copolymer.
 2. The surface-modified superabsorbent polymer ofclaim 1, wherein the water-soluble polyvalent cationic salt is containedin an amount of 0.001 to 5.0 parts by weight, based on 100 parts byweight of a superabsorbent resin.
 3. The surface-modified superabsorbentpolymer of claim 1, wherein the water-soluble polyvalent cationic saltcomprise a cationic ion selected from the group consisting of Al³⁺,Zr⁴⁺, Sc³⁺, Ti⁴⁺, V⁵⁺, Cr³⁺, Mn²⁺, Fe³⁺, Ni²⁺, Cu²⁺, Zn²⁺, Ag⁺, Pt⁴⁺,Au⁺, and a combination thereof, and an anion selected from the groupconsisting of sulfate (SO₄ ²⁻), sulfite (SO₃ ²⁻), nitrate (NO³⁻),metaphosphate (PO³⁻), phosphate (PO₄ ³⁻), and a combination thereof. 4.The surface-modified superabsorbent polymer of claim 3, wherein thewater-soluble polyvalent cationic salt is aluminum sulfate (Al₂(SO₄)₃)or zirconium sulfate (Zr(SO₄)₂).
 5. The surface-modified superabsorbentpolymer of claim 1, wherein the polycarbonic acid-based copolymer iscontained in an amount of 0.001 to 5.0 parts by weight, based on 100parts by weight of the superabsorbent polymer.
 6. The surface-modifiedsuperabsorbent polymer of claim 1, wherein the polycarbonic acid-basedcopolymer contains an alkoxypolyalkylene glycol mono(meth)acrylic acidester monomer and a (meth)acrylic acid monomer.
 7. The surface-modifiedsuperabsorbent polymer of claim 6, wherein the alkoxypolyalkylene glycolmono(meth)acrylic acid ester monomer is represented by the followingChemical Formula 1:

wherein, R¹ is a hydrogen atom or methyl; R²O represents an oxyalkylenemoiety of 2 to 4 carbon atoms; R³ is alkyl of 1 to 4 carbon atoms; and mis an integer of 50 to 200, expressing an average addition mole numberof oxyalkylene.
 8. The surface-modified superabsorbent polymer of claim6, wherein the alkoxypolyalkylene glycol mono(meth)acrylic acid estermonomer is at least one selected from the group consisting ofmethoxypolyethylene glycol mono(meth)acrylate, methoxypolypropyleneglycol mono(meth)acrylate, methoxypolybutylene glycolmono(meth)acrylate, methoxypolyethylene glycol polypropylene glycolmono(meth)acrylate, methoxypolyethylene glycol polybutylene glycolmono(meth)acrylate, methoxypolypropylene glycol polybutylene glycolmono(meth)acrylate, methoxypolyethylene glycol polypropylene glycolpolybutylene glycol mono(meth)acrylate, ethoxypolyethylene glycolmono(meth)acrylate, ethoxypolypropylene glycol mono(meth)acrylate,ethoxypolybutylene glycol mono(meth)acrylate, ethoxypolyethylene glycolpolypropylene glycol mono(meth)acrylate, ethoxypolyethylene glycolpolybutylene glycol mono(meth)acrylate, ethoxypolypropylene glycolpolybutylene glycol mono(meth)acrylate, and ethoxypolyethylene glycolpolypropylene glycol polybutylene glycol mono(meth)acrylate.
 9. Thesurface-modified superabsorbent polymer of claim 6, wherein the(meth)acrylic acid monomer is represented by the following ChemicalFormula 2:R²—COOM¹  [Chemical Formula 2] wherein, R² is an unsaturated hydrocarbonof 2 to 5 carbon atoms; and M¹ is a hydrogen atom, a monovalent ordivalent metal, an ammonium group, or an organic amine group.
 10. Thesurface-modified superabsorbent polymer of claim 6, wherein the(meth)acrylic acid monomer is at least one selected from the groupconsisting of an acrylic acid, a methacrylic acid, a monovalent ordivalent metal salt thereof, an ammonium salt thereof, and an organicamine salt thereof. 11-17. (canceled)
 18. A method for preparing asurface-modified superabsorbent polymer, comprising: a) providing asuperabsorbent polymer; b) pre-treating the superabsorbent polymer ofstep a) by mixing a water-soluble polyvalent cationic salt in an amountof 0.001 to 5.0 parts by weight, based on 100 parts by weight of thesuperabsorbent polymer provided in step a); and c) surface treating thepre-treated superabsorbent polymer of step b) by mixing a polycarbonicacid-based copolymer in an amount of 0.001 to 5.0 parts by weight, basedon 100 parts by weight of the superabsorbent polymer provided in stepa).
 19. A method for preparing a surface-modified superabsorbentpolymer, comprising: a) providing a superabsorbent polymer; b) preparinga mixture solution comprising a water-soluble polyvalent cationic saltin an amount of 0.001 to 5.0 parts by weight and a polycarbonicacid-based copolymer in an amount of 0.001 to 5.0 parts by weight, basedon 100 parts by weight of the superabsorbent polymer provided in stepa); and c) surface treating the superabsorbent polymer of step a) withthe mixture solution of step b).
 20. (canceled)
 21. The method of claim18, wherein the water-soluble polyvalent cationic salt comprise acationic ion selected from the group consisting of Al³⁺, Zr⁴⁺, Sc³⁺,Ti⁴⁺, V⁵⁺, Cr³⁺, Mn²⁺, Fe³⁺, Ni²⁺, Cu²⁺, Zn²⁺, Ag⁺, Pt⁴⁺, Au⁺, and acombination thereof, and an anion selected from the group consisting ofsulfate (SO₄ ²⁻), sulfite (SO₃ ²⁻), nitrate (NO³⁻), metaphosphate(PO³⁻), phosphate (PO₄ ³⁻), and a combination thereof.
 22. The method ofclaim 18, wherein the water-soluble polyvalent cationic salt is aluminumsulfate (Al₂(SO₄)₃) or zirconium sulfate (Zr(SO₄)₂).
 23. The method ofclaim 18, wherein the polycarbonic acid-based copolymer contains analkoxypolyalkylene glycol mono(meth)acrylic acid ester monomer and a(meth)acrylic acid monomer. 24-28.
 29. The method of claim 19, whereinthe water-soluble polyvalent cationic salt comprise a cationic ionselected from the group consisting of Al³⁺, Zr⁴⁺, Sc³⁺, Ti⁴⁺, V⁵⁺, Cr³⁺,Mn²⁺, Fe³⁺, Ni²⁺, Cu²⁺, Zn²⁺, Ag⁺, Pt⁴⁺, Au⁺, and a combination thereof,and an anion selected from the group consisting of sulfate (SO₄ ²),sulfite (SO₃ ²⁻), nitrate (NO³⁻), metaphosphate (PO³⁻), phosphate (PO₄³⁻), and a combination thereof.
 30. The method of claim 19, wherein thewater-soluble polyvalent cationic salt is aluminum sulfate (Al₂(SO₄)₃)or zirconium sulfate (Zr(SO₄)₂).
 31. The method of claim 19, wherein thepolycarbonic acid-based copolymer contains an alkoxypolyalkylene glycolmono(meth)acrylic acid ester monomer and a (meth)acrylic acid monomer.